The major objective of this research is to delineate the effects of delayed presentation of intravenous stimulant drugs on self- administration behavior. Squirrel monkeys will be trained to make an operant response which will produce intravenous infusion of cocaine, d- amphetamine, methylphenidate, or phenmetrazine. Different periods of delay will be scheduled between the reinforced response and the initiation of the drug infusion. Response rate under different values of delay will be studied. Other variables that may modulate the effects of delayed drug infusion will also be investigated. These include special stimuli present during the delay, and chained behavioral activity required during the delay. Different schedules of presentation for initiating the delay will be investigated, as well as gradual lengthening of the delay rather than abrupt introduction of long delays. Additional research will also determine how the reinforcing effect of these drugs is affected by different rates of infusion.